1. Field of the invention
The present invention relates to a reciprocating compressor for compressing gas such as air, and more particularly to a reciprocating compressor having a cylinder provided with a gas passage for passing compressed gas therethrough.
2. Description of the Related Art
FIG. 5 shows a conventional reciprocating compressor. Referring to FIG. 5, a piston 33 having a plurality of piston rings 36 on an outer circumferential surface thereof is adapted to be vertically reciprocated in a cylinder 31. When the piston 33 is lowered, gas is sucked from a suction valve 37 into the cylinder 31. Thereafter, when the piston 33 is lifted to come near a top dead center, the compressed gas in the cylinder 31 is discharged from a discharge valve 38 to the outside.
To reduce sliding friction between outer circumferential surfaces 47 of the piston rings 36 and an inner circumferential surface 41 of the cylinder 31 during vertical reciprocation of the piston 33 without using lubricating oil (i.e., in case of what is called an oil free type), the piston rings 36 are made of synthetic resin having a heat resistance and a low coefficient of friction, such as polytetrafluoroethylene (tetrafluoride resin) polychlorotrifluoroethylene (trifluoride resin).
The above-mentioned synthetic polymeric fluoride resins have a low coefficient of friction, but also have a low wear resistance. In particular, the amount of wear of the piston rings 36 greatly increases with an increase in sliding surface pressure of the outer circumferential surfaces 47 of the piston rings 36 against the inner circumferential surface 41 of the cylinder 31.
The piston rings 36 are engaged with ring grooves 35 formed on an outer circumferential surface 43 of the piston 33, and they are elastically projected from ring grooves 35 outwardly in a radial direction of the piston 33. During a gas compression stroke of the piston 33, the compressed gas of high pressure in the cylinder 31 penetrates from an upper surface of the piston 33 through a gap between the outer circumferential surface 43 of the piston 33 and the inner circumferential surface 41 of the cylinder 31 into the ring grooves 35. The compressed gas having entered the ring grooves 35 acts as a back pressure depicted by arrows 45 against inner circumferential surfaces 48 of the piston rings 36 so as to radially outwardly urge the piston rings 36. The back pressure against the inner circumferential surfaces 48 of the piston rings 36 is increased near the top dead center to further increase the sliding surface pressure of the outer circumferential surfaces 47 of the piston rings 36 against the inner circumferential surface 41 of the cylinder 31. Accordingly, the wear of the piston rings 36 made of the above-mentioned synthetic resin becomes remarkable. As a result, the durability of the piston rings 36 is reduced, and the synthetic resin powdered by the wear is mixed with the compressed gas in the cylinder 31 to contaminate the compressed gas.